Electro-optic frequency shifters, or modulators, have been built with almost 100 percent efficiency. Conversions over 50 percent require the interaction of a circularly polarized high power microwave beam traveling synchronously with a circularly polarized laser beam in an electro-optic crystal. We describe the inhomogeneous quarter-wave transformer structures made for a CO2 laser beam modulator with a conversion efficiency in excess of 80 percent. Conventional impedance matching synthesis was adapted to include the requirements for maintaining circular polarization and handling high power. Measured results are presented.